Lower mass main-sequence stars have higher mean density than higher mass ones, and Proxima Centauri is no exception: it has a mean density of 47.1 ×10 3 kg/m 3 (47.1 g/cm 3), compared with the Sun's mean density of 1.411 ×10 3 kg/m 3 (1.411 g/cm 3). The mass has been calculated directly, although with less precision, from observations of microlensing events to be 0.150 +0.062 The star's mass, estimated from stellar theory, is 12.2% M ☉, or 129 Jupiter masses ( M J). Because its distance is known, the actual diameter of Proxima Centauri can be calculated to be about 1/7 that of the Sun, or 1.5 times that of Jupiter. In 2002, optical interferometry with the Very Large Telescope (VLTI) found that the angular diameter of Proxima Centauri is 1.02 ☐.08 mas. More than 85% of its radiated power is at infrared wavelengths. Its total luminosity over all wavelengths is only 0.16% that of the Sun, although when observed in the wavelengths of visible light the eye is most sensitive to, it is only 0.0056% as luminous as the Sun. Its absolute visual magnitude, or its visual magnitude as viewed from a distance of 10 parsecs (33 ly), is 15.5. The M5.5 class means that it falls in the low-mass end of M-type dwarf stars, with its hue shifted toward red-yellow by an effective temperature of ~3,000 K. Proxima Centauri is a red dwarf, because it belongs to the main sequence on the Hertzsprung–Russell diagram and is of spectral class M5.5. Plot C shows variation over a 6.8 year period, which may be the length of the star's magnetic activity period. Plot B shows the relative brightness variation over the course of the star's 83 day rotation period. Plot A shows a superflare which dramatically increased the star's brightness for a few minutes. Three visual band light curves for Proxima Centauri are shown. A sub-Earth, Proxima Centauri d, orbits roughly 0.029 AU (4.3 million km) away every 5.1 days. A candidate super-Earth, Proxima Centauri c, orbits roughly 1.5 AU (220 million km) away every 1,900 d (5.2 yr). Proxima b orbits within Proxima Centauri's habitable zone-the range where temperatures are right for liquid water to exist on its surface-but, because Proxima Centauri is a red dwarf and a flare star, the planet's habitability is highly uncertain. Its estimated mass is at least 1.07 times that of Earth. Proxima Centauri b orbits the star at a distance of roughly 0.05 AU (7.5 million km) with an orbital period of approximately 11.2 Earth days. Proxima Centauri has two known exoplanets and one candidate exoplanet: Proxima Centauri b, Proxima Centauri d and the disputed Proxima Centauri c. The internal mixing of its fuel by convection through its core, and Proxima's relatively low energy-production rate, mean that it will be a main-sequence star for another four trillion years. The star's magnetic field is created by convection throughout the stellar body, and the resulting flare activity generates a total X-ray emission similar to that produced by the Sun. Although it has a very low average luminosity, Proxima Centauri is a flare star that randomly undergoes dramatic increases in brightness because of magnetic activity. Its actual diameter is about one-seventh (14%) the diameter of the Sun. Because of Proxima Centauri's proximity to Earth, its angular diameter can be measured directly. Proxima Centauri is a red dwarf star with a mass about 12.5% of the Sun's mass ( M ☉), and average density about 33 times that of the Sun. It is currently 12,950 AU (0.2 ly) from AB, which it orbits with a period of about 550,000 years. Proxima Centauri is a member of the Alpha Centauri star system, being identified as component Alpha Centauri C, and is 2.18° to the southwest of the Alpha Centauri AB pair. With a quiescent apparent magnitude of 11.13, it is too faint to be seen with the unaided eye. It was discovered in 1915 by Robert Innes and is the nearest-known star to the Sun. Its Latin name means the 'nearest of Centaurus'. Proxima Centauri is a small, low-mass star located 4.2465 light-years (1.3020 pc) away from the Sun in the southern constellation of Centaurus. False color Hubble Space Telescope WFPC2 image taken in 2013.
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